1. Field of the Invention
This invention relates to a bone strength measuring instrument and method for diagnosing the bone strength in part of a human body by using an ultrasonic wave, and more particularly to provide improvements in precision, reliability and accuracy.
2. Description of Related Art
Recent development and a need for practical use in recent years in the medical profession has encouraged the application of bone strength measuring instruments for measuring bone strength in part of a human body such as a wrist and a heel using an ultrasonic wave. For example, a technique has been known that, as shown in JP Patent No. 2863886, transmission or reception of an ultrasonic wave is conducted through a part of a human body in a state where a pair of ultrasonic probes each including an ultrasonic transmit/receive transducer holds a part of a human body therebetween and signals obtained in this situation are processed to thereby obtain information on the bone such as bone strength and bone density.
In a case where a bone strength of a person is measured, the measurement has generally been conducted on measurement sites such as a wrist or heel thereof, whereas it has been known that much of a cancellous bone exists in a specific site of a human body such as the wrist or heel and which is dissimilar to other sites on the human body thereof. FIG. 15 shows a cross-sectional structure of a wrist 1 as a model in which a cancellous bone 2 is at the center and a cortical bone 3 and a soft tissue 4 are disposed almost concentrically.
A bone strength measuring instrument described in JP Patent No. 2863886 has a construction obtained based on finding that if a measurement site where a cancellous bone exists is irradiated with an ultrasonic wave pulse in a direction of the diametral thereof, the ultrasonic wave is separated into two components different in acoustic speed from each other. To be detailed, an acoustic speed of an ultrasonic wave transmitted through the cancellous bone differs according to a different bone layer arrangement of the cancellous bone, that is between transmission through a porous structure portion and a bone marrow structure portion. As a result, an ultrasonic wave emitted from one ultrasonic probe is separated into two acoustic waves different in speed from each other, that is a first wave higher in acoustic speed and a second wave lower in acoustic speed, which are received by the other ultrasonic probe at different times. For example, an ultrasonic wave is separated into a first wave (a fast wave) and a second wave (a slow wave) in the cancellous bone and the acoustic speed of the first wave depends on specific properties and conditions of the bone. In JP Patent No. 2863886, there is available an advantage that both bone strength and bone quantity can be simultaneously obtained, which previously had been obtained independently of each other in conventional methods.
In a conventional bone strength measuring instrument, a prescribed procedure has been conducted based on preconditions described below in order to obtain thickness values, and properties and conditions of all tissues in wrist 1 as shown in FIG. 15, that is a cancellous bone 2, a cortical bone 3 and a soft tissue 4, respectively.
FIG. 16 schematically shows a construction of a measuring system in a conventional bone strength measuring instrument measuring bone strength present in the interior of a wrist 1 having the above structure, wherein numerical symbols 5 and 6 indicate ultrasonic probes set so as to be in close contact with both sides in the diametral direction of the wrist 1, the body of which, though not shown in a detailed manner, includes: a ultrasonic transmit/receive transducer and an acoustic impedance matching liquid (usually, water is used as the liquid and hereinafter referred to simply as matching water). The ultrasonic probes 5 and 6 are constructed so as to be freely movable linearly in a direction of moving closer to each other or away from each other, that is in a direction, left to right. The ultrasonic probes 5 and 6 are connected to an operation display (not shown) through cables. Note that provided as an operation display is a display unit capable of color display of a measured result and the like and a printer capable of color outputting of a measured result and like.
In FIG. 16, alphabetical symbols a, b, c, d, e and f indicate points on boundaries between the soft tissue 4, the cortical bone 3 and the cancellous bone 2 with which the shortest straight line 7 connecting the ultrasonic probes 5 and 6 intersects sequentially in that order. A symbol Laf indicates a distance between the ultrasonic probes 5 and 6 (hereinafter referred to simply as an inter-probe distance) and Lab and Lef are thickness values of the soft tissue 4, Lbc and Lde are thickness values of the cortical bone 3 and Lcd is a thickness value of the cancellous bone 2.
In a conventional practice, if a speed of an ultrasonic wave emitted from the ultrasonic probe 5 (or 6) is almost constant in the soft tissue 4 and if speeds in the soft tissue portions Lab and Lef is indicated with Vab and Vef than by definition, Vab=Vef=a determined value.
If a speed of the ultrasonic wave is almost constant in the cortical bone 3 and speeds in the cortical bone portions Lbc and Lde is indicated with Vbc and Vde than by definition, Vbc=Vde=a determined value.
The ultrasonic wave is, as described above, separated in the cancellous bone 2 into the first wave (fast) and the second wave (slow), wherein a speed of the first wave depends on the properties and condition (structure) of a bone, while a speed of the second wave is almost constant. Therefore, if a speed of the second wave is indicated with Vs by definition,Vs=a determined value.
In a case where the ultrasonic wave probes 5 and 6 are disposed as shown in FIG. 16, a distance Laf therebetween is measured in advance andLaf=a known determined value.
Now, if in a state shown in FIG. 16, an ultrasonic wave pulse is emitted from both ultrasonic wave probes 5 and 6, a signal shown in FIG. 17(A) is obtained as a signal from waves reflected at the boundaries b and e between the soft tissue 4 and the cortical bone 3 and waves reflected at the boundaries c and d between the cortical bone 3 and the cancellous bone 2. In FIG. 17(A), Tb and Tc are arrival times of reflected waves received by the ultrasonic wave probe 5 and Te and Td are arrival times of reflected waves received by the ultrasonic wave probe 6. If an ultrasonic wave pulse is emitted from one ultrasonic wave probe, for example, the ultrasonic wave probe 5, a signal as shown in FIG. 17(B) is obtained as a transmitted wave.
In the reflected wave shown in FIG. 17(A), the thickness Lab of the soft tissue 4 is obtained from the arrival time Tb from the boundary b received by the ultrasonic wave probe 5. Since the arrival time Tb is a time required to go and return between the boundaries a and b, the thickness Lab is obtained by multiplying a time required for one way Tb/2 by the acoustic speed Vab. That is,Lab=(Tb/2)×Vab. 
In a similar way, the thickness Lef of the soft tissue 4 is obtained from the arrival time Te from the boundary e received by the ultrasonic wave probe 6 andLef=(Te/2)×Vef. 
In the reflected wave, a thickness Lbc of the cortical bone 3 is obtained using a difference (Tc−Tb) between an arrival time Tc from the boundary c and an arrival time Tb from the boundary b by the ultrasonic wave probe 5. That is, since the difference (Tc−Tb) is a time required for an ultrasonic wave to go and return between the boundaries b and c, a thickness Lbc of the cortical bone 3 is obtained by multiplying a time for one way (Tc−Tb)/2 by the acoustic speed VbcLbc=(Tc−Tb)/2×Vbc 
In a similar way, a thickness Lde of the cortical bone 3 is obtained using a difference (Td−Te) between an arrival time Td from the boundary d and an arrival time Te from the boundary e by the ultrasonic wave probe. That is, since the difference (Td−Te) is a time required for an ultrasonic wave to go and return between the boundaries e and d, a thickness L be of the cortical bone 3 is obtained by multiplying a time for one way (Td−Te)/2 by the acoustic speed VdeLde=(Td−Te)/2×Vde 
Since all of Laf, Lab, Lef, Lbc and Lde are known as described above, a thickness Lcd of the cancellous bone 2 can be obtained. That is, Lcd=Laf−Lab−Lef−Lbc−Lde.
With the above operation conducted, there can be obtained thickness values of all the tissues including the soft tissue 4, the cortical tissue 3 and the cancellous bone 2.
As understood from the above description, in a conventional bone strength measuring apparatus, thickness values of the cortical bone 3 and the cancellous bone 2 are calculated using arrival times Tc and Td of reflected waves from the boundaries c and d between the cortical bone 3 and the cancellous bone 2. However, the inventors have found through research, it is not necessarily easy to detect the reflected waves and rather difficult to do so in some case. Therefore, according to the above calculation method, there has been a cases where thickness values of the cortical bone 3 and the cancellous bone 2 and the like cannot be obtained with good precision, with the result that in such a case, there cannot be obtained certain information on properties and a condition of a bone quantity, a bone strength or the like.
In a conventional bone strength measuring instrument, there has been a case where thickness values of tissues cannot be necessarily obtained with certainty at a site where a cancellous bone 2 does not exist, for example a wrist or a heel, with the result that no reliable information is obtained on the properties and conditions such as bone quantity and bone strength.
Thus there exists a need in instruments for measuring bone density to provide improvements in precision, reliability and accuracy to meet the demands of an aging population.